Method of forming semiconductor device with increased unit density

A method includes: abutting a first logic cell having a first cell height to a first memory cell having the first cell height; forming a first conductive rail and a second conductive rail at opposite sides of the first memory cell, respectively; forming a plurality of first conductive rails between the first conductive rail and the second conductive rail; forming a third conductive rail and a fourth conductive rail at opposite sides of the first logic cell, respectively; and forming a plurality of second conductive rails between the third conductive rail and the fourth conductive rail. An amount of the plurality of second conductive rails is larger than an amount of the plurality of first conductive rails.